Variable-leaf milfoil (Myriophyllum heterophyllum)
Photo credit: © Thomas Palmer/Flickr

Variable-Leaf Milfoil

(Myriophyllum heterophyllum)

Report this Species!

If you believe you have found this species in its invasive range in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

Variable-leaf milfoil, also called broadleaf watermilfoil or two-leaf milfoil, is a submerged perennial aquatic herb that is usually found rooted in up to 1.8 m (6 ft) of water. It gets its name from its two noticeably different forms of submerged and emergent leaves.

Identification

Leaves: Submerged leaves, usually 2-6 cm (0.8-2.4 in) in length, are finely dissected into 7-11 thread-like leaflets, which are arranged in very dense whorls of 4-5, giving it a bottlebrush appearance. In general, there is less than 5 mm (0.2 in) between whorls. Color ranges from reddish to greenish-brown. Emergent leaves that may not appear until late summer are bright green, oval, narrow (0.6 cm [0.2 in] wide), and stand 15-20 cm (6-7.9 in) out of the water.

 

Flowers: Emergent flower spikes are 5-30 cm (2-12 in) long with whorls of four flowers each. Petals are small, measuring less than 3 mm (0.1 in), and emerge on green to reddish stalks from June to September.

 

Fruits/Seeds: Small and almost perfectly round fruits have four chambers and a rough surface.

 

Stems/Roots: Stout, usually dark red to reddish-brown stems measure up to 3 mm (0.1 in) in diameter and 100 cm (39 in) in length.

Similar Species

Take caution because all milfoil species display a wide vegetative variability. Variable-leaf milfoil may be confused with parrotfeather (Myriophyllum aquaticum) and Eurasian watermilfoil (Myriophyllum spicatum); however, neither species has winter buds and they generally have a greater distance between whorls (1 cm [0.4 in] or greater).

Habitat

This extremely well-adapted plant can thrive in freshwater ponds, lakes, ditches, and other still or flowing aquatic systems, and even survives under ice.

Spread

Reproduction is primarily through vegetative fragments, which can hitchhike on boats, trailers, and fishing equipment. It may also reproduce via seed production, but probably to a lesser extent.

Distribution

While native to the southeastern United States, variable-leaf milfoil has become invasive in most of the northeast. In Pennsylvania, it is considered native and endangered in Erie County, but shows invasive tendencies in lakes and impoundments in Fayette, Monroe, and Bucks counties.

 

Note: Distribution data for this species may have changed since the publication of Pennsylvania's Field Guide to Aquatic Invasive Species (Second Edition 2015), the source of information for this description.

*To learn more about the native/invasive range of variable-leaf milfoil in Pennsylvania, check out the information available for this species on our Invasive Here but Not There page.

Environmental Impacts

Variable-leaf milfoil can alter ecosystems by forming dense mats that shade out native aquatic plants, inhibit water flow,and impede recreational activities.

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